1. Field of the Invention
The present invention relates to an air conditioning apparatus for electric automobiles capable of varying heating performance by controlling the number of rotations of a refrigerant compressor.
2. Description of the Related Art
In an air conditioning apparatus for electric automobiles, since a heat source for heating cannot be sufficiently secured as in gasoline automobiles, a heat-pump-type refrigerating cycle, in which a cooling operation and heating operation are performed depending on the flow of the refrigerant, is used. Electric automobiles having no internal combustion engine are provided with an electric motor for driving a refrigerant compressor, and control the number of rotations of a refrigerant compressor by controlling the frequency of the electric motor by an inverter.
A method of controlling the number of rotations of the refrigerant compressor has been proposed in which the position of the temperature adjustment lever is adjusted by the vehicle's passenger to correspond to the inverter frequency (Refer to Japanese Patent Laid-Open No. 4-151318). In household air conditioners, the thermal load is determined according to the difference between the current room temperature and the desired room temperature. On the basis of this thermal load, the number of rotations of the refrigerant compressor is controlled.
However, in this method, since feedback control of the number of rotations of the refrigerant compressor is not based upon the actual blowout air temperature, it is not suitable for an air conditioning apparatus for electric automobiles in which blowout air directly strikes the passengers.
Accordingly, it is conceivable that even in the air conditioning apparatus for electric automobiles that an actual blowout air temperature can be detected by a temperature sensor as is performed in a conventional air conditioning apparatus for motor vehicles (an air conditioning apparatus installed in a motor vehicle having an internal combustion engine), and the number of rotations of the refrigerant compressor can be controlled by feedback control so that the detected value of the temperature sensor becomes a desired blowout air temperature (TAO) calculated on the basis of a set value.
In this method, however, since the heat capacity of a heat exchanger for heating is large and the response of the temperature sensor is poor (slow), when the number of rotations of the refrigerant compressor is increased to realize the desired blowout air temperature, the discharge pressure of the refrigerant compressor is increased too much before the desired blowout air temperature is reached. As a result, a high-pressure cut-off switch for detecting a high-pressure-side pressure of the refrigerant compressor is actuated, stopping the operation of the refrigerant compressor.
Since, as described above, the heat capacity of a heat exchanger for heating is large and the response of the temperature sensor is poor, the blowout air temperature with respect to the desired blowout air temperature TAO hunts (See FIG. 10). In particular, when the indoor air volume (air volume fed to a heat exchanger for heating) during a heating operation is varied or when the desired blowout air temperature is varied incident to the changes in the set temperature, the electric current of the inverter varies considerably (hunting), causing the discharge pressure of the refrigerant compressor to vary. Therefore, the blowout air temperature hunts greatly.